Currently, panels of carbon fiber reinforced polymers (CFRP) are stiffened with longitudinal reinforcing elements of different shapes, for example T, L, U or omega, manufactured by co-cured or co-bonded processes. Then, transversal reinforcing elements are assembled by rivets.
More specifically, the skin could be automatically laid up using, for instance, Automated Tape Laying (ATL) or Automated Fiber Placement (AFP) and every stringer could also be automatically laid up. This process has the drawback that one or two laminates are needed to get every longitudinal reinforcing element, increasing recurring costs and lead time.
Afterwards laminates are trimmed to a final longitudinal reinforcing element shape and every longitudinal reinforcing element is individually hot formed by using press forming machines and then the integration of the longitudinal reinforcing elements with the skin has to be performed. In a co-curing process, the integration operation may consist of locating the longitudinal reinforcing elements, for instance stringers, and the skin in a tool and additionally introducing a system for pressurizing the interior of the stringer in case of having a closed shape longitudinal reinforcing element. For a co-bonding process, a curing cycle of the stringers is performed previous to the step of locating the stringers into the tool. Another possibility would be firstly curing the skin and afterwards integrating the stringers.
Finally, the skin and stringers assembly is bagged and a curing cycle is applied. At the end, stringers and skin are demolded. Finally, the frames that are previously cured are assembled to the skin and stringers structure by rivets.
Additionally, crosses between longitudinal and transversal reinforcing elements are made such that only one of the two elements is maintained continuous. This implies that both elements shall be thereafter joined in the crossing zones by a plurality of connecting elements (riveted or bonded), resulting in high weight penalties and/or high production and assembly costs and also debonding risks when using connecting elements bonded to the beams and frames.
In composites, especially in the aeronautical industry, it is necessary to develop new concepts based in high strain and stiffness-weigh ratios. For this purpose, low thickness skins with stringers in different sections like “T”, “L” or “omega” are developed.
On the other hand, innovative skins concepts to be developed shall provide a cost saving compared with current configurations. For this purpose, manufacture of co-cured/co-bonded parts is very interesting as it decreases cured cycles and labor hours in assembly, due to removal of riveting. However, it is a challenge to solve the composites core with efficient stiffness and easy manufacturing.